Tumor suppressor gene LKB1 is frequently inactivated by mutations in lung cancer, which is often associated with concomitant activating KRas mutation. LKB1 inactivation is positively associated with advanced tumor progression and metastasis. While it is well-documented that LKB1 regulates cancer cell growth mainly through targeting AMPK/mTOR and p53 signaling pathways, the molecular mechanisms underlying the regulation of tumor invasion and metastasis by LKB1 are poorly understood. Interestingly, upon the loss of LKB1, ERK3, an atypical MAP kinase, is highly upregulated in KRasG12D-induced tumors and in human lung cancer cells. ERK3 was recently shown to be overexpressed in lung cancer and promote lung cancer cell migration and invasion by upregulating matrix metalloproteinase (MMP) gene expression. In contrast to the well-studied classic MAPKs, such as ERK1/2, little is known about the molecular regulation of ERK3 signaling (gene expression and kinase activation). Importantly, ingenuity pathway analysis of ERK3 interacting proteins identified in our pilot study shows that ERK3 is highly associated with TGF (or Wnt)/TAK1 signaling and RhoGDI signaling. In addition, preliminary data from in vitro co-culture experiments demonstrate that depletion of LKB1 in lung cancer cells facilitates the conversion of M?s to M2 type; M2 M?s then produce high levels of TGF1 and Wnt2 that may in turn act on tumors in activating TAK1 and ERK3. Based on these findings, we hypothesize that ERK3 signaling is upregulated upon the loss of LKB1 in lung tumor microenvironment; upregulation of ERK3 signaling then promotes lung tumor progression and metastasis. To test this hypothesis, the following specific aims are proposed. 1). To define the molecular regulation of ERK3 signaling: c-Jun-mediated gene expression and TAK-1-mediated kinase activation in response to TGF/Wnt signals, in cultured lung cancer cell systems. 2). To test in vivo the idea that ERK3 signaling is upregulated upon the loss of LKB1 in tumor cells through c-Jun-mediated ERK3 gene upregulation and TAK-1-mediated activation of ERK3 signaling pathway in response to the stimulation of TGFs and/or Wnts secreted by M2 type TAMs, utilizing a lung-specific KRasG12D/LKB1-deficient tumor model. 3). To determine the roles of ERK3 in lung tumor progression and metastasis by conditionally overexpressing ERK3 in KRasG12D lung tumor mouse model or conditionally knocking out ERK3 in lung tissue-specific KRasG12D/LKB1-null mouse model. The objective of this proposal is to elucidate ERK3 signaling pathway and to determine its role in lung tumor progression and metastasis. The proposed work is anticipated to define ERK3 as a novel diagnostic marker and/or a therapeutic drug target for the treatment of advanced lung cancer.